In a nitride semiconductor, a band gap, a dielectric breakdown electric field, and a saturated drift velocity of an electron are larger than those in silicone (Si), GaAs, and the like. Moreover, in a heterostructure transistor formed on a substrate having a main surface of a (0001) plane and made from AlGaN/GaN, two-dimensional electron gas (hereinafter also referred to as 2 DEG) is generated in a hetero interface by spontaneous polarization and piezo polarization.
Therefore, in the heterostructure transistor, 2 DEG having a sheet carrier concentration of at least approximately 1×1013 cm−2 can be obtained without doping. In recent years, a high electron mobility transistor that uses highly concentrated 2 DEG as a carrier has drawn attention, and various HEMT structure (heterojunction) field-effect transistors are proposed.
FIG. 6 is a diagram showing a cross-section structure of a conventional field-effect transistor 500 in Patent Literature (PTL) 1. Hereinafter, the field-effect transistor is also referred to as a FET. Moreover, the FET is hereinafter also referred to as a device.
As shown in FIG. 6, in the field-effect transistor 500, a first semiconductor layer 510 (operation layer) made of a first nitride semiconductor (GaN) and a second semiconductor layer 520 (barrier layer) made of a second nitride semiconductor are stacked on a substrate 501. A band gap of the second nitride semiconductor is larger than a band gap of the first nitride semiconductor.
The second semiconductor layer 520 is formed on the first semiconductor layer 510, so that a heterojunction interface is formed. As a result, in a region in the vicinity of the heterojunction interface in the first semiconductor layer 510, a 2 DEG layer 511 is formed.
In the second semiconductor layer 520, openings 521.1 and 521.2 penetrating the second semiconductor layer 520 to reach (be in contact with) the first semiconductor layer 510 are formed.
It is to be noted that each of the openings 521.1 and 521.2 is formed to penetrate the 2 DEG layer 511 and reach a region below the 2 DEG layer 511.
A conductive material is buried inside the opening 521.1, so that a source electrode S50 that is an ohmic electrode is formed. Moreover, a conductive material is buried inside the opening 521.2, so that a drain electrode D50 that is an ohmic electrode is formed.
On the second semiconductor layer 520, a gate electrode G50 that is a schottky electrode is formed. Moreover, the gate electrode G50 is formed between the source electrode S50 and the drain electrode D50.
It is to be noted that a surface protection film 550 is formed on the source electrode S50, the drain electrode D50, and the gate electrode G50.
With this structure, the ohmic electrode and the 2 DEG layer are directly contact with each other, thereby decreasing contact resistance in the ohmic electrode. Hereinafter, the field-effect transistor 500 is referred to as a conventional FET.